Either a project or a product the team would require a SCRUM master who actually does the "Project Controlling " part. Hence based on the project style the PMO members can get distributed among the SCRUM teams and take up roles like SCRUM MASter, Product owner or Retrospective Handler. But in all the cases, the involvment given by the PMO team in SCRUM methods would be more deeper and clearer than when you have performed in tradional methods. Can explain in detail if you have more questions.

I don't understand. The ScrumMaster is not supposed to do any "controlling" as far as I know. And I'm not aware of the role "Retrospective Handler" at all.